Having different credentials for different games is so yesterday, and Mojang knows this as they’ll soon be making it so you can sign into all their games with one username.
Of course, that wouldn’t mean much right now, as Minecraft is their only publicly available game. Instead, they’re launching the system with Cobalt sometime this month, and will be integrating it with Minecraft in early 2012. Naturally it will be integrated into Scrolls as well, whenever it happens to be released.
Apparently, the system works like Blizzard’s Battle.net, where you have one master account that can be used to log in to many smaller accounts, including multiple accounts for the same game.
It should be noted that Mojang isn’t just going to switch your current Minecraft account over to this system. You’ll have to opt in once it’s available, at least at first. It seems very likely there will be a mandatory switch at some point in the future.